Carbon monoxide (CO) is a colorless, tasteless, and odorless by-product gas that results from exogenous sources such as pollution, fires, or smoking. In the blood, CO combines readily with hemoglobin (Hb) in a reversible chemical reaction to produce carboxyhemoglobin (HbCO). Headaches, dizziness, nausea and other flu-like symptoms are often associated with HbCO levels around 10-15%. With higher HbCO levels between 15% and 40%, common symptoms include severe headaches, mental confusion, and fainting. When the HbCO levels in blood exceed 40%, unconsciousness or death may result.
Traditionally, HbCO has been measured by automated clinical analyzers in the laboratory. One common clinical analyzer is the IL-482 CO-Oximeter (manufactured by Instrumentation Laboratories, Lexington, Mass.). This oximeter is an automated instrument which analyzes a blood sample for total hemoglobin (THb), percent oxyhemoglobin (%HbO.sub.2), %HbCO, and reduced hemoglobin (%RHb) contents. (Brown L., "A New Instrument for the Simultaneous Measurement of Total Hemoglobin, % Oxyhemoglobin, % Carboxyhemoglobin, % Methemoglobin, and Oxygen Content in Whole Blood" Institute of Electrical and Electronic Engineering (IEEE) Transactions on Biomedical Engineering, 27:132-8(1980); and Instrumentation Laboratory, IL-482 CO-Oximeter Operator's Manual, Lexington, Mass.) Clinical analyzers, such as the IL-482, are best operated in a well equipped laboratory by highly trained personnel. These analyzers are usually used in a hospital setting because they are bulky, expensive, and powered by 110 V a.c. In addition, CO-Oximeters require a hemolyzed blood sample for analysis. For the reasons mentioned above, these clinical analyzers are not suitable for use by paramedics in the field.
Although several types of devices are currently available for detecting CO levels in ambient air, the definitive diagnosis of CO poisoning is made in the hospital by measuring HbCO levels in blood utilizing spectrophotometric means. HbCO and HbO.sub.2 both absorb less light in the red region of the spectrum (i.e. .about.660 nm) than RHb. Therefore, blood that contains elevated levels of CO possesses the same distinctive red color as blood that is highly oxygenated. For this reason, based on color alone, it is difficult to differentiate well oxygenated blood from blood containing high levels of CO.
Recently, non-invasive pulse oximeters for measuring arterial oxygen saturation (SaO.sub.2) in blood have become widely available. These pulse oximeters are popular devices for use by paramedics for rapid assessment of the oxygenation status of a patient. Although pulse oximeters can accurately measure SaO.sub.2, they do not provide direct measurements of HbCO levels in blood. In fact, in the presence of elevated levels of &gt;10% HbCO, pulse oximeters produce false readings since HbCO and HbO.sub.2 have similar optical absorption properties. (Mendelson Y., "Pulse Oximetry: Theory and Applications for Noninvasive Monitoring", Clin Chem;38(9) : 1601-7(1992))